metric system
System of weights and measures developed in France in the 18th century and recognized by other countries in the 19th century.
In 1960 an international conference on weights and measures recommended the universal adoption of a revised International System (Système International d'Unités, or SI), with seven prescribed ‘base units’: the metre (m) for length, kilogram (kg) for mass, second (s) for time, ampere (A) for electric current, kelvin (K) for thermodynamic temperature, candela (cd) for luminous intensity, and mole (mol) for quantity of matter.
Supplementary units
Two supplementary units are included in the SI system – the radian (rad) and steradian (sr) – used to measure plane and solid angles. In addition, there are recognized derived units that can be expressed as simple products or divisions of powers of the basic units, with no other integers appearing in the expression; for example, the watt.
Non-SI units
Some non-SI units, well established and internationally recognized, remain in use in conjunction with SI units: minute, hour, and day in measuring time; multiples or submultiples of base or derived units that have long-established names, such as tonne for mass and litre for volume; and specialist measures such as the metric carat for gemstones.
Prefixes
Prefixes used with metric units are tera (T), million million times; giga (G), billion (thousand million) times; mega, (M) million times; kilo (k), thousand times; hecto (h), hundred times; deca (da), ten times; deci (d), tenth part; centi (c), hundredth part; milli (m), thousandth part; micro (μ), millionth part; nano (n), billionth part; pico (p), trillionth part; femto (f), quadrillionth part; atto (a), quintillionth part.
| The metric system was made legal for most purposes in the USA in the 19th century. A Metric Act was passed in the USA in 1975 to promote the use, without compulsion, of the metric system. |
